This invention relates generally to multi-component cobalt alloy material which is used for sputter deposition of a magnetic layer on a data/storage disk, and more specifically to a method of producing an alloy material, such as that based on a brittle Co-based alloy, which exhibits microstructural and magnetic properties that are optimized for magnetron sputtering, and which finds application in the formation of a sputtering target.
The manufacture of storage disks and read/write heads involves the use of target materials that are used to sputter deposit a thin film media onto a suitable substrate. Cobalt-based alloy targets are frequently used for this purpose.
However, multi-component Co alloys with primary elemental additions such-as Cr, Pt, Ni (0 to 30 atomic%) and secondary elemental additions such as Ta, B, Nb, Sm, Fe, Si, Zr, W, Mo, V, Hf, and Ti (0 to 30 atomic%) can be very difficult or impossible to conventionally roll-process if the concentration of limited solid-solubility elements is excessive. Since vacuum induction melting is necessary to ensure a high purity product, the only practical way to manufacture these brittle alloys has been to make as-cast targets. However, as cast targets have several unfavorable microstructural characteristics such as large grain-size, gross segregation of matrix and precipitate phases, through thickness microstructural and chemical gradients and porosity.
It is fairly typical that a pair of magnetic alloy targets can be used to fabricate in excess of 20,000 individual data-storage disks. Since the magnetic target alloy is continually loosing surface atomic layers during the sputtering process, through thickness and in-plane target microstructural homogeneity is essential to ensure film property homogeneity on the many thousands of disks fabricated from each target and the many thousand more fabricated from the numerous targets constituting a production lot or originating from several individual production lots. A production lot represents all the targets that are exposed to exactly the same thermomechanical history (i.e. originating from one melted ingot).
Precipitate segregation in the matrix of Co-based magnetic target alloys has been shown to impact deposited film magnetic properties such as Coercivity and Overwrite. When tens of thousands of data storage devices are being made from several targets, it is necessary that the Coercivity response be consistent on all the disks, i.e., quality control, and not be a function of the specific target utilized. Therefore, there is a substantial need in the art for Co-based magnetic targets which exhibit consistent performance, both within a target and from target to target.
Prior work serves to illustrate the effect of target precipitate and grain uniformity on the sputter deposited film: Two targets of a Coxe2x80x94Crxe2x80x94Ta alloy were fabricated. Target A was fabricated using standard techniques and possessed a coarse and non-uniform Ta precipitate-phase microstructural morphology. Target B was fabricated to yield a homogeneous microstructure consisting of a uniform dispersion of the precipitate phase. Sputter process trials were in which the two targets, A and B, were placed on either side of the sputtering chamber so that they would be used for material deposition on the opposite sides of the same disk. These precautions were taken to ensure that exactly the same sputter conditions and testing conditions applied for films deposited using the two differently fabricated targets. Furthermore, targets A and B were interchanged in the sputtering chamber to ensure that no anomalies associated with location in the chamber were obscuring the results of the investigation. The results of the analysis revealed that the magnetic films on disks fabricated using target A exhibited coercivities that ranged from 1580 Oersteds to 1780 Oersteds. In contrast, the films on disks sputter deposited with magnetic material using target B exhibited Coercivities that ranged between 1920 to 2000 Oersteds. The film Coercivity was ascertained using conventional VSM testing techniques, widely employed in the disk manufacturing industry. There are several noteworthy points resulting from this analysis. First, target A, possessing a inhomogeneous microstructure, resulted in films with a significantly lower Coercivity response than films deposited using target B which possessed a homogeneous microstructure. Second, the actual film Coercivities obtained from target A (overall range=200 Oersteds) were,much less consistent than the film Coercivities obtained from target B (overall range=80 Oersteds). These results demonstrate that if target precipitate and grain uniformity are not controlled, the resulting Coercivity response of the sputtered film can be diminished and the disk-to-disk Coercivity consistency can be adversely effected.
Prior art which has been concerned with this or similar problems include the following United States Patents:
Accordingly, there is a need in at least the data storage industry, for a technique by which targets made of Co-based alloys or the like, which conventionally tend to exhibit brittleness to the degree that wrought processing can not be used, can be manufactured with the required microstructurally homogeneous, fully dense and low permeability characteristics necessary to enable the production of high quality storage disks. The present invention meets this need.
An object of the present invention is therefore to provide a method of forming a piece of material suitable for making a magnetron sputtering target, that is capable of being rolled and wrought processed in a manner which improves the physical and magnetic characteristics such as grain size, crystallographic texture, chemical/microstructural uniformity and permeability.
A further object of the present invention is to enable the formation of a sputtering target which is constituted in such a manner as to cause the magnetic layers which are sputter deposited onto the surface of a data/storage disk, to exhibit improved physical/magnetic characteristics.
During efforts to overcome the above mentioned problem and to develop a microstructurally homogeneous, fully dense and low permeability target, it was unexpectedly discovered that by subjecting cast ingots to an annealing step at temperatures between about 1500xc2x0 F. and 2500xc2x0 F., with or without the presence of an applied hydrostatic or compressive pressure, which can aid in stress assisted diffusional healing of porosity, particularly good microstructures were obtained.
The annealing could be typically carried out for 0.5-168 hours. When pressure was applied, the pressures of 2 to 60 ksi were typically used. It was noted that the dimensions of an ingot treated in the above manner, had an effect on the final product and that a thickness-to-width ratio less than about 0.5 was favorable to facilitate a successful conclusion to the rolling and wrought processing.
In brief, the above objects are thus achieved by forming an ingot of material which is normally too brittle to allow successful rolling and wrought processing so as to have a thickness-to-width ratio less than about 0.5, and annealing this ingot in a controlled temperature environment of 1500xc2x0 F. to 2500xc2x0 F., and then rolling the annealed member at an initial temperature range between 2500xc2x0 F. and 1000xc2x0 F., reheating the ingot if the plate temperature falls below 500xc2x0 F.